Various devices have been utilized over time for the separation of nitrogen and oxygen from air. Many such devices rely on a membrane that is exposed to pressurized air, such that oxygen molecules preferentially (compared to the larger nitrogen molecules) diffuse through the membrane, resulting in an oxygen-enriched gas on one side of the membrane and a nitrogen-rich gas on the other side of the membrane. These gases are also referred to as oxygen-enriched air (OEA) and nitrogen-enriched air (NEA), respectively. The effectiveness of membranes at performing the task of separating gases can be characterized by a trade-off that membranes experience between permeability of the membrane to the gas molecules targeted for diffusion across the membrane versus selectivity of the membrane between the targeted gas molecules and other molecules in the gas mixture. A plot of the collection of permeability versus selectivity values for various materials is known as a Robeson plot, and the upper performance limit of membrane materials is identified by a line along that plot known as the Robeson limit. Various types of materials have been used as membranes for gas separation. Inorganic metal oxides of various compositions and crystal structures have been proposed, but the materials are brittle and susceptible to damage and are also difficult to fabricate in membrane configurations. Various types of polymer and/or polymer composite materials have also been proposed. These materials can overcome some of the mechanical limitations of inorganic materials, but they typically rely on a membrane structure where selectivity is provided by a combination of the gas molecule solubility in the polymer matrix and its diffusivity through the polymer matrix, i.e. the torturous path that the gas molecules must traverse through in order to cross the membrane, and may not provide a Robeson limit that is as high as desired. Attempts to increase the selectivity of composites by incorporating high-selectivity materials into a polymer matrix have met with limited success because polymer matrices configured to prevent gas molecules from bypassing the dispersed selective material component also tend to limit the overall permeability of the membrane. Moreover, in most of the cases, these highly selective materials are incompatible with the polymer matrix, which leads to voids in the composite and reduction in selectivity.
There are, of course, many uses for OEA or NEA, so there are a variety of applications for devices that separate oxygen and nitrogen, including but not limited to medical oxygen concentrators, atmospheric oxygen supplementation systems, and NEA-based combustion suppression systems. In recent years, commercial and other aircraft have been equipped with fuel tank suppression systems that introduce NEA into a fuel tank headspace or ullage, often by bubbling NEA through the liquid fuel. Such systems require NEA with a nitrogen concentration of at least 90% by volume, and attempt to minimize payload weight and size while maintaining target NEA output across a wide variety of operating conditions. Nitrogen-generating using membrane technology has been used and proposed for use in these and other systems; however many of these systems suffer from various shortcomings such as performance specification limitations imposed by the membrane's Robeson limit, lack of stability in performance specifications over time, inability to maintain performance levels across a wide variety of conditions, inability to meet payload weight or size requirements, etc. Accordingly, there continues to be a need for new approaches to the separation of nitrogen and oxygen.